docs: add five modern logo concepts for McpMux app

Design logo concepts reflecting core app identity:
- Hub & Routes: central hub with radiating connections (multiplexer)
- Gateway Shield: security shield with converging data flows
- Lightning M: bold lightning bolt with network nodes
- Convergence Arrows: many-to-one flow visualization
- Abstract Nexus: orbital rings with central core

All use the brand palette (cyan #00D9FF, gold #FFD700) on dark backgrounds.

https://claude.ai/code/session_012dsFytvfPhFE5U9GuZ8Fy7

Author: claude